On the structures and luminescence properties of Eu3+ -doped Li2 CaGeO4 , Ca2 GeO4 , and Ca5 Ge3 O11 compounds.

ABSTRACT

Powder samples of Li2 CaGeO4 , Ca2 GeO4 , and Ca5 Ge3 O11 doped by 0.5, 1, 2, 3, 4 and 5 at% Eu3+ relative to the Ca2+ , were prepared using a conventional solid-state synthesis technique. X-ray diffraction (XRD) analyses confirmed obtaining the pure phases at all dopant concentrations. In parallel, single crystals of the three compounds with the experimentally found optimal Eu3+ concentration were grown using a flux method. Structural investigation on the single crystals were done with a special attention to the form of the Ca-O polyhedron, the mean Ca-O distance, the Ca-Ca distance in the structure, the distortion degree of the polyhedron, as well as the Eu-Ca substitution site. The main spectral characteristics were analyzed and several relationships between the structural and spectra features were found. The optimal dopant concentration was 3 at% for Ca2 GeO4 and 4at% for Ca5 Ge3 O11 and Li2 CaGeO4 . Commission Internationale de l'éclairage coordinates of the samples showed emission colours in the red region close to the standard red coordinates and slightly influenced by the active ion concentration. The obtained results showed that europium-doped Li2 CaGeO4 , Ca2 GeO4 , and Ca5 Ge3 O11 could be used as red phosphors.